It is well known that expansion and contractions of muscle produce electrical signals that circulate upon the surface of a person's skin. Perhaps the most common are the expansions and contractions of the cardiac muscle, which are typically referred to as ECG signals. These ECG signals exhibit particular waveforms containing several distinct characteristics for each heartbeat. These characteristics, generally labeled P, Q, R, S and T, according to common medical usage, have allowed medical science to monitor a person's heartbeat or heartbeat count.
Of the three positive peaks of a single heartbeat signal, the P, R and T pulses, it is usually the R peak that is the largest. Since it is necessary that ony one peak be detected for each heartbeat, a threshold detector can be employed in the simple case to distinguish between P and T waves, on the one hand, and R waves on the other. Accordingly, the R-wave peaks are available to trigger the threshold detector to generate heartbeat count and are often so used.
At times, however, the P and/or T waves are taller than the R waves. In this instance utilization of a simple comparator operating directly on the unfiltered ECG waveform becomes erratic. One attempt at controlling this problem is be employing some form of filtering to attenuate the P and T waves in relation to the R waves, since R waves contain higher frequencies than other parts of the ECG waveform. Thus, for example, high-pass filters are used to attenuate P and T waves more than R waves. However, this solution is not always satisfactory. Additionally, pulse width discrimination is also used to detect the R wave and identify the ECG waveform from other muscle activity.
However, as noted at the outset, all muscle tissue will emit electrical signals when expanding or contracting, the heart being only one of the many muscle groups of a person's body. Since other muscle groups will also emit electrical signals, it is desirable that the person remain relatively motionless while an ECG waveform is being obtained--particularly if the fidelity of the desired ECG waveform is to be as accurate as possible.
When a person is in motion, however such as when exercising, the problem of monitoring the person's heartbeat becomes extremely difficult; the reason being that sensors placed on the person detecting the ECG signal also receive electrical signals produced by the other expanding and contracting muscles of the body--and other motion signals--termed "artifact". The heretofore known practices of limiting the frequency response characteristics of the ECG waveform and/or rejecting artifact by amplitude discrimination or pulse width discrimination to identify each heartbeat has been found to be generally insufficient, even when these techniques are used in combination.
Therefore, it is desirable that additional techniques be found to allow accurate monitoring of a person's heartbeat during exercise to allow one to obtain reliable heartbeat information in the presence of such artifact. This is particularly true if the person wishes to exercise his or her cardiovascular system, yet keep his or her heartbeat within predetermined limits.